In his otherwise very excellent (and I should say indispensable) commentary on Monadology, Lloyd Strickland claims that Leibniz’s Mill it is an argument from inconceivability, viz: If there is no conceivable way perception is mechanical, then it is not. Strickland himself points out that the crucial term “conceivability” never occurs in the text:
Supposing that there were a machine whose structure produced thought, sensation, and perception, we could conceive of it as increased in size with the same proportions until one was able to enter into its interior, as he would into a mill. Now, on going into it he would find only pieces working upon one another, but never would he find anything to explain perception. It is accordingly in the simple substance, and not in the compound nor in a machine that the perception is to be sought.
But Leibniz gives us a middle term: the Mill has only pieces working/pushing upon one another. The mill as such starts when some part is pushed by the outside (a river, the wind, whatever) and it ends when it no longer pushes on something. Therefore the mill as such mediates a force. All machines mediate, covert, etc. They are pure conduits that, as Newton would put it, trade velocity for force. The mill argument therefore seems to be:
A mechanical action is of itself intermediary and not final.
Perception an action that is final and not of itself an intermediary.